Well maybe someone sealed the barrel without knowing someone was one in there. Then they met their doom here in breakneck and this person never got the chance to escape...or maybe this boat was owned by pirates and this was how they handled stowaways...

[ She shuddered at the thought They had a real pirate in their crew after all, and thinking about physical torture made her skin crawl. Urgh.
Cynthia gently returns the skull into the barrel then drags the thing to the top of the deck where she drops it onto the water. The guy deserved a proper burial at sea, at least. ]